Structural homology among mammalian and Saccharomyces cerevisiae isoprenyl-protein transferases

Farnesyl-protein transferase (FTase) purified from rat or bovine brain is an α/β heterodimer, comprised of subunits having relative molecular masses of approximately 47 (a) and 45 kDa (β). In the yeast Saccharomyces cerevisiae, two unlinked genes, RAM1/DPR1 (RAMl) and RAM2, are required for FTase activity. To explore the relationship between the mammalian and yeast enzymes, we initiated cloning and immunological analyses. cDNA clones encoding the 329-amino acid COOH-terminal domain of bovine FTase α-subunit were isolated. Comparison of the amino acid sequences deduced from the α-subunit cDNA and the RAM2 gene revealed 30% identity and 58% similarity, suggesting that the RAM2 gene product encodes a subunit for the yeast FTase analogous to the bovine FTase α-subunit. Antisera raised against the RAMl gene product reacted specifically with the β-subunit of bovine FTase, suggesting that the RAM1 gene product is analogous to the bovine FTase β-subunit. Whereas a ram1 mutation specifically inhibits FTase, mutations in the CDC43 and BET2 genes, both of which are homologous to RAMl, specifically inhibit geranylgeranyl-protein transferase (GGTase) type I and GGTase-II, respectively. In contrast, a ram2 mutation impairs both FTase and GGTase-I, but has little effect on GGTase-II. Antisera that specifically recognized the bovine FTase a-subunit precipitated both bovine FTase and GGTase-I activity, but not GGTase-II activity. Together, these results indicate that for both yeast and mammalian cells, FTase, GGTase-I, and GGTase-II are comprised of different but homologous β-subunits and that the α-subunits of FTase and GGTase-I share common features not shared by GGTase-II.